Field of the Invention
This invention relates to flow field plate constructions for solid polymer electrolyte fuel cells.
Description of the Related Art
Solid polymer electrolyte or proton exchange membrane fuel cells (PEMFCs) electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, an anode flow field plate and a cathode flow field plate, each comprising numerous fluid distribution channels for the reactants, are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications. In such a stack, the anode flow field plate of one cell is thus adjacent to the cathode flow field plate of the adjacent cell. For assembly purposes, a set of anode flow field plates is often bonded to a corresponding set of cathode flow field plates prior to assembling the stack. A bonded pair of an anode and cathode flow field plates is known as a bipolar plate assembly. In some constructions, bipolar plates may be made from a single piece of material (e.g. carbon) in which the cathode and anode flow fields are formed on opposite sides of the material. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated on the electrochemically inactive surfaces of the flow field plates of the cells in the stacks (i.e. the coolant flow field is located within a bipolar plate assembly).
To provide both reactants and the coolant to and from the individual cells in the stack, a series of ports are generally provided at opposing ends of the individual cells such that when the cells are stacked together they form manifolds for these fluids. Further required design features then are passageways in the plates to distribute the bulk fluids in these formed manifolds to and from the various channels in the reactant and coolant flow fields in the plates. These passageway regions are referred to as the transition regions. The transition regions can themselves comprise numerous fluid distribution channels, e.g. oxidant and/or fuel transition channels.
While simple seeming enough in principle, achieving a desirable distribution of reactants to the electrodes and a desirable removal of by-products therefrom in high power density fuel cell designs is nonetheless quite complex and many factors have to be considered. For instance, the landings which separate the fluid distribution channels in typical flow field plates provide mechanical support and thus cannot be too thin. However, the distribution of gases to and from those regions in the gas diffusion layers immediately adjacent the landings is not as good as that in those regions immediately adjacent the channels.
Another issue to consider is that the compositions of the supplied reactant gases changes significantly as they travel through a practically sized fuel cell. As reactants get consumed and gaseous and liquid water by-products are created, these gas compositions change substantially and designs have to be incorporated to accommodate the changing nature of these gases.
Much effort has gone into improving the gas distribution to and from the fuel cell electrodes and in understanding the details of flow within such fuel cell stacks. For instance, convective flow in gas diffusion layers and different flow field designs have been discussed in publications such as C. Y. Soong et al., “Analysis of reactant gas transport in a PEM fuel cell with partially blocked fuel flow channels”, Journal of Power Sources, 143 (2005) 36-47; and T. Kanezaki et al., “Cross-leakage flow between adjacent flow channels in PEM fuel cells”, Journal of Power Sources, 162 (2006) 415-425.
In particular, consideration has been given to designs which include projections or the like in the flow field channels and which provide various benefits. JP2004241141 for instance includes projections inside the gas channels and turbulence occurs in the reaction gas which spreads with an effect that generation of electricity improves. In US 20040151973, flow restrictors are strategically located in the flow field channels to achieve certain desired pressure differentials. Further, JP2004327162 effectively includes restrictions in order to obtain more uniform surface pressure in the cells.
However, while such designs can offer modest benefits for gas distribution or other purposes under certain operating conditions, there is a continuing need for improved reactant gas distribution under other and varied operating conditions. This invention addresses such issues and provides further related advantages.